Numerous types of thermal spray coating methods and systems are known in the prior art. In general, such methods comprise the deposition of a wire or powdered material onto a surface to be coated. In one particular process, known as electric-arc (two wire) spray coating, two consumable wires form electrodes of an electric arc or "arc ball". The two wires are electrically energized and converge at a point in which the electric arc is formed. A stream of compressed atomizing gas is passed through the converging point to atomize the molten material and drive a molten metal particle stream formed by the electric arc along an axis forward of the converging zone.
Various prior patents discuss electric-arc spray systems, noteworthy of which include U.S. Pat. Nos. 1,968,992 (apparatus for coating surfaces), 2,610,092 (spray discharge nozzle), 4,464,414 (method for spraying metallic coatings), 4,992,337 (electric arc spraying of reactive metals); 5,066,513 (method of producing titanium nitride coatings by electric arc thermal spray); 4,937,417 (metal spraying apparatus); 4,98,557 (method of arc spraying); 4,986,477 (spray gun with adjustment of the shape of the jet); 4,992,337 (electric arc spraying of reactive metals); 5,017,757 (pulsed arc welding machine); 5,109,150 (open-arc plasma wire spray method and apparatus); 5,143,139 (spray deposition method and apparatus); 5,145,710 (method and apparatus for applying a metallic coating to threaded end sections or plastic pipes and resulting pipe); 5,148,990 (adjustable arc spray and rotary stream sprinkler); 5,191,186 (narrow beam arc spray device), 5,194,304 (method of thermally spraying solid lubricant onto a metal target), 5,442,153 (high velocity electric-arc spray apparatus and method of forming materials); 5,466,906 (process for coating automotive engine blocks) and 5,468,295 (apparatus and method for thermal spray coating of interior surfaces).
More specifically, of the above listed patents, U.S. Pat. No. 5,468,295 to Marantz describes a thermal spray coating apparatus, such as a two wire arc apparatus. The nozzle contains a plurality of pores facing generally inwardly towards a coating material particle stream, such as an atomized molten metal stream of a two-wire arc thermal spray apparatus. The ports sequentially receive a deflecting gas flow, such that the direction moves circumferentially about the axis of the particle stream. The deflecting gas entrains the coating materials and carries it radially to the surface of the part to be coated or the nozzle assembly. When such nozzle is inserted into an engine bore, it is described as radially coating the bore, on its surface.
However, a number of problems exist with the prior art device of Marantz that have been overcome by the present invention First, as a pneumatic device it is less responsive than the electro-mechanical device herein described, and more cumbersome and bulky with-multiple air passages. In addition, by sequencing or strobing the air stream, the Marantz device will tend to overlap the coating layers, whereas the apparatus herein describe provides a true continuous stream coating which can vary over a wide range of rotation rates. Moreover, valving, i.e. switching from one tube to an adjacent tube, is non-linear. Accordingly, it is very difficult by such process to provide a smooth transition from full flow on one set tube to the next. In addition, the lift of the arc ball in the Marantz device must be substantial. This translates into some instability in the radial flow which is further complicated by the comparative size of the orifices which would have a tendency to cause chatter (i.e., intermittent extinguishing of the arc and reignition thereof), and a focused spray (or narrowed pattern) is impossible.
In addition, one pervasive problem with all thermal-arc spraying devices of the prior art, rotatable or otherwise, is that air flow from the air knife has been found to create a negative pressure between the wicket (the area immediately behind the consumable electrodes), and the knife base, as the air flows away from the knife. This negative pressure is believed to be responsible for drawing material from the arc ball and depositing such material onto the knife.
Accordingly, it is a first object of this invention to overcome the aforesaid problems of prior art thermal spray devices and provide a rotating arc spray gun wherein the only part of the gun head that rotates is the atomizing air jet.
More specifically, it is an object of the present invention to overcome the aforesaid problems of the prior art thermal spray devices and provide a rotating arc spray gun wherein a continuous stream coating can be applied over a wide range of rotation rates, and which avoids a sequencing, strobing or pulsed air stream.
In addition, the present invention has as a more specific object the preparation of a rotating arc spray gun wherein a deflecting valve assembly (or air knife) rotates about an arc ball formed by two consumable electrodes and wherein the deflecting valve assembly contains a plurality of ports providing a semi-circular pattern thereby providing a hooped shaped air flow around the arc and a focused radial delivery of an atomized metal coating.
Finally, the present invention has as its object the installation of what can be described as a negative pressure compensator means, in an arc gun containing a deflecting valve assembly (rotatable or stationary). The negative pressure compensator eliminates any negative pressure formed by the effect of the air flow from the deflecting valve assembly as the air flows away from said assembly, thereby maintaining the arc ball in the proper alignment position for efficient coating.